Isoforms of nitric oxide synthase. Properties, cellular distribution and expressional control

Antisense Oligonucleotides Targeting Protein Kinase C-α, -βI, or -δ But Not -η Inhibit Lipopolysaccharide-Induced Nitric Oxide Synthase Expression in RAW 264.7 Macrophages: Involvement of a Nuclear Factor κB-Dependent Mechanism

The signaling pathway for protein kinase C (PKC) activation and the role of PKC isoforms in LPS-induced nitric oxide (NO) release were studied in RAW 264.7 macrophages. The tyrosine kinase inhibitor genestein attenuated LPS-induced NO release and inducible nitric oxide synthase (iNOS) expression, as did the phosphoinositide-specific phospholipase C (PI-PLC) inhibitor U73122 and the phosphatidylcholine-specific phospholipase C (PC-PLC) inhibitor D609. LPS stimulated phosphatidylinositol (PI) hydrolysis and PKC activity in RAW cells; both were inhibited by genestein. The PKC inhibitors (staurosporine, calphostin C, Ro 31-8220, or Go 6976) or long-term 12-O-tetradecanoylphorbol 13-acetate (TPA) treatment also resulted in inhibition of LPS-induced NO release and iNOS expression. Western blot analysis showed expression of PKC-α, -βI, -δ, -η, and -ζ in RAW cells; down-regulation of PKC-α, -βI, and -δ, but not -η, was seen after long-term TPA treatment, indicating the possible involvement of one or all of PKC-α, -βI, and -δ, but not -η, in LPS-mediated effects. Treatment with antisense oligonucleotides for these isoforms further demonstrated the involvement of PKC-α, -βI, and δ, but not -η, in LPS responses. Stimulation of cells with LPS for 1 h caused activation of NF-κB in the nuclei by detection of NF-κB-specific DNA-protein binding; this was inhibited by genestein, U73122, D609, calphostin C, or antisense oligonucleotides for PKC-α, -βI, and -δ, but not -η. These data suggest that LPS activates PI-PLC and PC-PLC via an upstream tyrosine kinase to induce PKC activation, resulting in the stimulation of NF-κB DNA-protein binding, then initiated the expression of iNOS and NO release. PKC isoforms α, βI, and δ were shown to be involved in the regulation of these LPS-induced events.

Nitric oxide and prostaglandin E2 formation parallels blood-brain barrier disruption in an experimental rat model of bacterial meningitis

During meningitis, the host produces a plethora of signaling agents as part of a coordinated defense mechanism against invading pathogens. Nitric oxide (NO) and prostaglandin E2 (PGE2) are two such inflammatory mediators produced in response to bacterial endotoxins. Disruption of the blood-brain barrier (BBB) is one of many pathophysiological consequences of meningitis. The present objective was to examine the time-course of NO and PGE2 production in relationship to BBB permeability alterations during experimentally-induced meningitis. Meningeal inflammation was elicited by intracisternal administration of the bacterial endotoxin, lipopolysaccharides (LPS; 200 microg), and NO, PGE2, and BBB integrity were monitored over the next 24 h. Meningeal NO production was assessed by headspace chemiluminescence; cerebrospinal fluid PGE2 was determined by enzyme-linked immunosorbent assay (ELISA) immunoassay; and BBB integrity was determined by the brain accumulation of 14C-sucrose. Similar time-course profiles for NO and PGE2 were observed, with a peak effect for both inflammatory mediators observed within 6-8 h after intracisternal LPS dosing. Statistically significant (p < 0.05) disruption of the BBB was observed in various brain regions. Strikingly similar temporal relationships were observed for NO and PGE2 production and BBB disruption. These results suggest the hypothesis that NO and PGE2 may act in conjunction to disrupt the BBB during experimental meningitis.

Erratic behavior of nitric oxide within the immune system: illustrative review of conflicting data and their immunopharmacological aspects

The literature data assembled in this article document the variation of immunobiological effects of nitric oxide (NO). A number of factors are obviously responsible for the diversity, ranging from inactivity, alleviation, but not rarely to exacerbation of certain pathogenetic processes. A better understanding of NO interactions with the immune system can only be reached if more complex experimental designs to study the effects of reactive nitrogen species are adopted in the future. They should integrate major participating variables and take into account pharmacodynamic/kinetic aspects of NO production in triggering the ultimate effects. If manipulation of NO in the organism by means of recently developed NO inhibitors and NO donors is to become a rational tool of immunopharmacological strategies, detailed knowledge of their pharmacologies and toxicologies is urgently needed in order to differentiate between the effects of NO and other side effects. Hopefully, this approach could improve the predictability of the clinical outcomes of NO manipulation.

Stimulation of macrophages by Bacillus firmus: production of nitric oxide and cytokines

Immunostimulatory properties of gram-positive Bacillus firmus were investigated under in vitro conditions using murine peritoneal macrophages. B. firmus stimulated in a concentration and time dependent manner the secretion of tumour necrosis factor-alpha (TNF-alpha) and interleukin-10 (IL-10), but it had no influence upon interferon-gamma (IFN-gamma) and interleukin-2 (IL-2) production. It also substantially augmented production of nitric oxide (NO) induced by exogenous IFN-gamma. Inhibitory experiments using neutralizing antibodies against TNF-alpha and/or IL-10 have demonstrated that these cytokines are responsible for triggering the underlying mechanism(s) leading to enhanced NO production. The cytokine-stimulatory and NO-costimulatory properties could participate in the antiinfectious and anticancer effects of B. firmus, detected previously in the in vivo experiments.

Modulation of white adipose tissue lipolysis by nitric oxide

In isolated adipocytes, the nitrosothiols S-nitroso-N-acetyl-penicillamine (SNAP) and S-nitrosoglutathione stimulate basal lipolysis, whereas the nitric oxide (NO.) donor 1-propamine, 3-(2-hydroxy-2-nitroso-1-propylhydrazine) (PAPA-NONOate) or NO gas have no effect. The increase in basal lipolysis due to nitrosothiols was prevented by dithiothreitol but not by a guanylate cyclase inhibitor. In addition the cyclic GMP-inhibited low Km, cyclic AMP phosphodiesterase activity was inhibited by SNAP suggesting that SNAP acting as NO+ donor increases basal lipolysis through a S-nitrosylation mediated inhibition of phosphodiesterase. Contrasting with these findings, SNAP reduced both isoproterenol-stimulated lipolysis and cyclic AMP production, whereas it failed to modify forskolin-, dibutyryl cyclic AMP-, or isobutylmethylxanthine-stimulated lipolysis, suggesting that SNAP interferes with the beta-adrenergic signal transduction pathway upstream the adenylate cyclase. In contrast with SNAP, PAPA-NONOate or NO gas inhibited stimulated lipolysis whatever the stimulating agents used without altering cyclic AMP production. Moreover PAPA-NONOate slightly reduces (30%) the hormone-sensitive lipase (HSL) activity indicating that stimulated lipolysis inhibition by NO. is linked to both inhibition of the HSL activity and the cyclic AMP-dependent activation of HSL. These data suggest that NO. or related redox species like NO+/NO- are potential regulators of lipolysis through distinct mechanisms.

Neuronal nitric-oxide synthase interaction with calmodulin-troponin C chimeras

Calmodulin (CaM) binding activates neuronal nitric-oxide synthase (nNOS) catalytic functions and also up-regulates electron transfer into its flavin and heme centers. Here, we utilized seven tight binding CaM-troponin C chimeras, which variably activate nNOS NO synthesis to examine the relationship between CaM domain structure, activation of catalytic functions, and control of internal electron transfer at two points within nNOS. Chimeras that were singly substituted with troponin C domains 4, 3, 2, or 1 were increasingly unable to activate NO synthesis, but all caused some activation of cytochrome c reduction compared with CaM-free nNOS. The magnitude by which each chimera activated NO synthesis was approximately proportional to the rate of heme iron reduction supported by each chimera, which varied from 0% to approximately 80% compared with native CaM and remained coupled to NO synthesis in all cases. In contrast, chimera activation of cytochrome c reduction was not always associated with accelerated reduction of nNOS flavins, and certain chimeras activated cytochrome c reduction without triggering heme iron reduction. We conclude: 1) CaM effects on electron transfer at two points within nNOS can be functionally separated. 2) CaM controls NO synthesis by governing heme iron reduction, but enhances reductase activity by two mechanisms, only one of which is associated with an increased rate of flavin reduction.

Erectile dysfunction in aging: upregulation of endothelial nitric oxide synthase

OBJECTIVES

To evaluate whether alterations in nitric oxide (NO) synthesis or activity contribute to age-related erectile dysfunction and to elucidate the mechanisms causing these alterations using the rabbit as our model of aging.

METHODS

We compared the ability of the rabbit cavernosal smooth muscle to relax in the organ bath in response to acetylcholine (Ach, endothelium-dependent vasodilator), sodium nitroprusside (SNP, an NO donor), and A23187 (a calcium ionophore) in young (6 month old) and aged (2.5 to 3.5 year old) rabbits. In addition, the immunohistochemical expression of endothelial nitric oxide synthase (eNOS) in both young and aged rabbit cavernosal tissue was examined. Endothelial integrity was examined immunohistochemically with JC70.

RESULTS

Ach-mediated relaxation of penile corporal tissue was significantly attenuated from a maximum of 68.39 +/- 6.27 (0.1 mM Ach, n = 4) in young rabbits to 39.02 +/- 4.88 (0.1 mM Ach, n = 6) in aged rabbits (P < 0.04). No statistically significant difference (P > 0.05) was noted between cavernosal relaxation to sodium nitroprusside between young rabbits (97.8%, 0.1 mM SNP, n = 5) and aged rabbits (76.1%, 0.1 mM SNP, n = 5). This suggested that the defect in the Ach-NO pathway was at the level of NO synthesis, not activity. Immunohistochemical staining for eNOS demonstrated upregulation in both the vascular endothelium and corporal smooth muscle of aged rabbit tissue compared with young rabbit cavernosal tissue (n = 5). Anatomic endothelial integrity was demonstrated in the young and aged rabbits by the presence of JC70. This suggested that the defect in the Ach-NO synthetic pathway was not at the level of eNOS and was not due to anatomic endothelial cell disruption. Finally, Ach-mediated cavernosal smooth muscle relaxation in the young rabbit was not significantly augmented (P > 0.05) in the presence of the calcium ionophore A23187 (10 microM). A23187, however, significantly augmented (P < 0.04) Ach-mediated relaxation in the aged rabbit from a maximum of 33.93 +/- 6.58 to 41.55 +/- 6.58 (10 microM Ach, n = 5). This suggested that a potential defect in the Ach-NO synthetic pathway was at the level of intracellular calcium flux and possibly at the level of the calcium-eNOS interaction.

CONCLUSIONS

Endothelium-dependent relaxation is attenuated in the aging rabbit; eNOS is upregulated in the aging rabbit; and no difference is noted in response to direct NO donation between the young and aged rabbit. The endothelium is anatomically intact in both the young and aging rabbit. The calcium ionophore A23187 augmented the attenuated vasorelaxation in the aging rabbit cavernosum (although not to the levels seen in the young rabbit cavernosum) and had no effect on the young rabbit cavernosum. These data suggest that erectile dysfunction in the aging rabbit cavernosum appears to be related to endothelial dysfunction and is characterized by eNOS upregulation and aberrant intracellular calcium fluxes.

In vivo nitric oxide detection in the septic rat brain by electron paramagnetic resonance

To detect nitric oxide (NO) in the rat brain during lipopolysaccharide (LPS)-induced sepsis, electron paramagnetic resonance (EPR) was employed with the NO trapping technique, using an iron and N,N-diethyldithiocarbamate (DETC) complex. An X-band (about 9.5 GHz) EPR system detected a triplet signal (g = 2.038) derived from an NO-Fe-DETC complex being superimposed on the g(perpendicular) signal of Cu-DETC complex at liquid nitrogen temperature. The height of the triplet signal peaked seven hours after injection of 40 mg/kg of LPS, and over 25 x 10(4) U/kg of IFN-gamma enhanced the LPS-induced NO formation. Pretreatment with N(G)-monomethyl-L-arginine (NMMA), an NO synthase inhibitor, deleted only the triplet signal. A triplet signal (g(iso) = 2.040, aN = 1.28 mT) derived from the NO-Fe-DETC complex was also observed at ambient temperature. Then, a home-built 700 MHz EPR system was used to detect an NO signal in the septic rat brain in vivo. We successfully monitored the NO-Fe-DETC signal in the head region of a living rat under the condition that provided maximum height of the NO-Fe-DETC signal in the X-band EPR study. Pretreatment with NMMA again deleted the NO-Fe-DETC signal. This is the first EPR observation of endogenous NO in the brain of living rats.

Effect of 17beta-oestradiol on cytokine-induced nitric oxide production in rat isolated aorta

1. Studies were performed on isolated aortic rings without endothelium to investigate the effect of 17beta-oestradiol on cytokine-induced nitric oxide production by the inducible nitric oxide synthase (iNOS). 2. Treatment of the isolated aortic rings with interleukin-1beta (IL-1beta, 20 micro ml(-1)) led to the expression of iNOS mRNA and protein, as well as significant nitrite accumulation in the incubation media and suppression of phenylephrine (1 nM-10 microM)-evoked contraction. 3. Cycloheximide (1 microM), a protein synthesis inhibitor, prevented iNOS protein expression, nitrite accumulation and the suppression of contractility by IL-1beta on the isolated aortic rings. 17Beta-oestradiol (1 nM-10 microM) and the partial oestrogen receptor agonist 4-OH-tamoxifen (1 nM-10 microM) produced concentration-dependent inhibition of IL-1beta-induced nitrite accumulation and restored vasoconstrictor responsiveness to phenylephrine, similar to the iNOS inhibitor aminoguanidine (100 microM). 4. Semiquantitative PCR demonstrated decreased iNOS mRNA in the IL-1beta-induced and 17beta-oestradiol-treated rings. Western blot analysis of rat aorta homogenates revealed that 17beta-oestradiol treatment resulted in a reduction in IL-1beta-induced iNOS protein level. 5. Incubation with tumour necrosis factor alpha (TNF alpha, 1 ng ml(-1)) resulted in significant nitrite accumulation in the incubation media and suppression of the smooth muscle contractile response to phenylephrine, similar to IL-1beta. The effects of TNF alpha were also inhibited by co-incubation of the rings with 17beta-oestradiol and 4-OH-tamoxifen (1 microM). 6. The anti-transforming growth factor-beta1 (TGF-beta1) antibody, which inhibited TGF-beta1-induced suppression of nitrite production from IL-1beta-treated vascular rings, did not affect the inhibitory action of 17beta-oestradiol, suggesting that the effect of oestrogen on iNOS inhibition was not mediated by TGF-beta1. 7. These results show that the ovarian sex steroid, 17beta-oestradiol is a modulator of cytokine-induced iNOS activity in rat vascular smooth muscle and its mechanism of action involves decrease of iNOS mRNA and protein.

Pharmacological characterization of endothelial cell nitric oxide synthase inhibitors in isolated rabbit aorta

Different receptors mediating the release of endothelium-derived nitric oxide (EDNO) have been identified at endothelial level. In the present study we aimed to characterise, on rabbit aorta by means of pharmacological tools, the generation of EDNO by receptors located on endothelial cell membrane (M3, P2u, P2y) and by direct activation of Ca2+ entry into the endothelial cell. Four vasodilating drugs were tested (acetylcholine, UTP, A23187 and 2-methyl-thio-ATP); they were active only if the endothelial layer was intact, suggesting that they act through endothelial receptors. The effect of different nitric oxide synthase (NOS) inhibitors (0.1 mM: L- and D-NAME, L-NMMA, L-NIO and 7-NI) was investigated on NO-mediated relaxation induced by the relaxants in vessels with intact endothelium. NOS inhibitors differently affected relaxation mediated by the vasoactive drugs in isolated rabbit aorta. Reversibility of the inhibition by using a fixed concentration of L-arginine (0.1 mM) was different depending on the relaxing drug and NOS-inhibitor. The data obtained support the coexistence in aortic vessel of more than one endothelial cell NOS isoform, each provided with different receptor coupling.

Effect of endothelium removal on prostaglandin and nitric oxide function in pulmonary resistance arteries in the lamb

We have recently shown that isolated pulmonary resistance arteries of the fetal lamb have prostaglandin (PG) I2 based and nitric oxide (NO) based relaxing mechanisms, which are activated by oxygen (at neonatal levels) and bradykinin. The present study was carried out to ascertain whether these mechanisms remain operational after removal of the endothelium. Endothelium-denuded vessels pre-equilibrated at a neonatal PO2 were not affected by indomethacin (2.8 µM), while they contracted weakly to NG-nitro-L-arginine methyl ester (L-NAME, 100 µM). However, the latter response did not reach significance and resembled that of intact vessels at fetal PO2. Bradykinin (0.1-100 nM) dose dependently (from 1-3 nM upwards) relaxed endothelium-denuded arteries that had been precontracted with a thromboxane (TX) A2 analog (ONO-11113, 0.1 µM) or excess potassium (5 mM Ca2+ in K+-Krebs) at a neonatal PO2. The response was the same under the two conditions, but it was smaller than that of intact arteries. Bradykinin relaxation of ONO-11113-contracted arteries was completely or nearly completely inhibited by indomethacin and L-NAME. We conclude that endothelium-denuded, pulmonary resistance arteries maintain PG (conceivably PGI2) mediated and NO-mediated relaxing mechanisms. These extra-endothelial mechanisms are activated by bradykinin but not by oxygen.Key words: perinatal pulmonary circulation, endothelium, nitric oxide, prostaglandin, oxygen.

ZR-75-1 human breast cancer cells: expression of inducible nitric oxide synthase and effect of tamoxifen and phorbol ester on nitric oxide production

The existence of the L-arginine-nitric oxide pathway was investigated in ZR-75-1 human breast cancer cells. The presence of inducible nitric oxide synthase in these cells was confirmed by staining with an anti-iNOS antibody. ZR-75-1 cells spontaneously produced nitric oxide (NO) and this production could be significantly (P < 0.001) enhanced by L-arginine (0.01-10 mM) and was inhibited by L-NAME (2 mM). Stimulating the cells with phorbol 12-myristate 13-acetate (PMA) (200-1000 nM) resulted in a significant (P < 0.001) increase in NO2- secreted into the medium. Although treatment of the same cells with tamoxifen (10(-10)-10(-6) M) had no effect on NO production, tamoxifen was able to significantly (P < 0.001) downregulate PMA-enhanced nitrite production. Our results suggest that tamoxifen could play a role in the biology of nitric oxide in breast tumours.

Calcium and protein kinase C mediate high-glucose-induced inhibition of inducible nitric oxide synthase in vascular smooth muscle cells

Abnormal vascular smooth muscle (VSMC) proliferation is a key feature in diabetes-associated atherosclerotic disease. Since nitric oxide inhibits VSMC tone, migration, adhesion, and proliferation, we examined the effects of high glucose on IL-1beta-induced NO release from VSMCs in culture. Confluent smooth muscle cells, preincubated with either 5 mmol/L (mM) or 20 mmol/L (mM) glucose for 48 hours, were stimulated with IL-1beta. Nitrite was measured in the culture medium after 24 hours. IL-1beta-induced a 15-fold increase in NO production in normal glucose medium. Glucose (10 to 30 mmol/L (mM)) significantly reduced the response to IL-1beta. High glucose (20 mmol/L (mM)) inhibited IL-1beta-evoked NO production by approximately 50%. IL-1beta-stimulated [3H] citrulline-forming activity of the nitric oxide synthase (NOS) was also significantly lower in high-glucose-exposed cells, and this was reflected in diminished cellular levels of NOS protein. To assess the role of protein kinase C (PKC), membrane PKC activity was measured, and glucose (20 mmol/L (mM)) significantly increased it. Immunoblotting of the membranes revealed a glucose-induced increase in the PKC betaII isoform. 1,2-Dioctanoyl-glycerol, a PKC activator, mimicked the high-glucose effect on IL-1beta-induced NO release, while staurosporine, a PKC inhibitor, reversed it. The role of calcium in the glucose-mediated inhibition of cytokine-induced NO release was determined by treatment with BAPTA, an intracellular chelator of calcium. BAPTA partially reversed the inhibitory effects of glucose. Increasing intracellular calcium by A23187, an ionophore or thapsigargin, an inhibitor of endoplasmic reticulum Ca2+-ATPase, significantly decreased IL-1beta-induced NO release and NOS expression. These results indicate that glucose-induced inhibition of IL-1beta-stimulated NO release and NOS expression may be mediated by PKC activation and increased intracellular calcium.

Expression of nitric oxide synthase-2 in glia associated with CNS pathology

This chapter discusses the expression of nitric oxide synthase-2 (NOS-2) in glia associated with central nervous system (CNS) pathology. The production of nitric oxide (NO) in the nervous system is catalyzed by three, highly homologous isoforms of NO synthase (NOS). NOS-2, the dimeric, heme-containing, soluble protein whose activity is independent of a rise in intracellular calcium, is variously termed ‘inducible,’ ‘immunologic,’ and ‘macrophage NOS (macNOS).’ Nitric oxide inhibits not only NOS-2 activity but also regulates the level of NOS-2 messenger RNA (mRNA) expression through a mechanism involving NF-^K B. There is specific evidence for the glial expression of NOS-2 associated with neuronal injury and infection of the CNS and in neurodegenerative and demyelinating diseases. Direct injury in the CNS results in a reactive gliosis, characterized by the induction of the glial fibrillary acidic protein gene and changes in astrocyte morphology.

Colocalization of demethylating enzymes and NOS and functional effects of methylarginines in rat kidney

NG-monomethylarginine (L-NMA) and asymmetric NG, NG-dimethylarginines (ADMA) are endogenous inhibitors of cellular L-arginine uptake and/or nitric oxide (NO) synthesis that are implicated in renal parenchymal and Dahl salt-sensitive hypertension. Since the L-arginine:(L-NMA + ADMA) ratio determines NO synthase (NOS) activity, we compared the immunohistochemical distribution of NOS with NG, NG-dimethylarginine dimethylaminohydrolase (DDAH), which inactivates dimethylarginines (DMA) and L-NMA by hydrolysis to L-citrulline. Neuronal NOS (nNOS) was expressed predominantly in tubular epithelial cells of macula densa (MD), endothelial NOS (eNOS) in vascular endothelial cells (EC), and inducible NOS (iNOS) quite widely in tubular epithelium, including proximal tubules (PT), thick ascending limbs of Henle (TAL), distal convoluted tubule and intercalated cells (IC) of the collecting duct. Immunostaining for DDAH was present in PT, TAL, MD, and IC, and was also present in the glomerulus, Bowman's capsule, and endothelium of blood vessels. DDAH was detected in small vesicles of TAL and PT by electron microscopic (EM) immunocytochemistry. To study the effects of methylarginines on tubuloglomerular feedback (TGF) response, vehicle or methylarginines (10(-3) M) were added to artificial tubular fluid (ATF) perfused orthogradely from the late PT at 40 nl. min-1 while assessing changes in glomerular capillary pressure from proximal stop flow pressure (PSF). Whereas the maximal TGF responses were unchanged by vehicle (delta TGF 0 +/- 0%) or symmetric DMA (SDMA; +1 +/- 2%, NS), they were enhanced by L-NMA (+22 +/- 4%, P < 0.001) and asymmetric DMA (ADMA; +28 +/- 3%, P < 0.001). Since L-arginine transport can regulate renal epithelial NO generation, methylarginines (10(-3) M) or vehicle were co-perfused orthogradely with [3H]-L-arginine from the late PT and collected at the early distal tubule to study arginine uptake from the perfused loop of Henle. All methylarginines reduced fractional loop [3H] absorption significantly (P < 0.001; vehicle, 84 +/- 6; ADMA, 49 +/- 6; SDMA, 56 +/- 6; L-NMA, 41 +/- 6%). In conclusion, sites of DDAH expression in the vasculature or nephron are all sites of expression of an isoform of NOS. L-NMA, ADMA, and SDMA all inhibit renal tubular L-arginine uptake, whereas L-NMA and ADMA, but not SDMA, enhance TGF responses. Therefore, DDAH may regulate the cellular L-arginine: methylarginine levels in specific renal cells, thereby governing cell-specific L-arginine uptake and NO generation in renal tubular epithelium.

Nitric oxide synthase activity in tissues of the bovine eye

BACKGROUND

Nitric oxide synthase (NOS) is present in many ocular tissues where it may have different physiological functions. This warrants a thorough characterization of NOS activity in the eye.

METHODS

NOS distribution and its biochemical properties were determined in the retina, choroid, ciliary processes (CP), and trabecular meshwork (TM).

RESULTS

Retinal NOS required NADPH (diphenylene-iodonium, a flavoprotein inhibitor, which inhibited enzyme activity with an IC50 of 0.36 microM, FAD (40 microM), FMN (40 microM), and BH4 (4 microM) as cofactors for optimal activity. Ocular NOS appeared to be regulated by free divalent cations, since its activity was inhibited by EDTA (slopes > 3.0 and IC50 values of 12.8, 19.7, and 53 microM, respectively). Ocular NOS required calmodulin, since NOS activity was inhibited by trifluoperazine (calmodulin inhibitor, IC50 = 41 microM). NOS activity is widely distributed in the eye, (choroid > retina > CP > TM) and is mainly cytosolic (70-95%). L-Arginine analogs inhibited NOS in the retina, choroid, and TM. In all three tissues, NG-methyl-L-arginine displayed the highest affinity for inhibition (IC50 = 0.2-0.7 microM) followed by canavanine (IC50 = 13-33 microM), while aminoguanidine only weakly inhibited NOS (IC50 = 93-179 microM).

CONCLUSION

In all tissues, the order of potency of inhibition points to the presence of constitutive rather than inducible NOS. Moreover, it is possible that TM contains more than a single form of NOS.

Influence of endotoxin on contractility in the rat gastric fundus

The influence of in vivo treatment with E. coli lipopolysaccharide endotoxin on the contractility of the rat gastric fundus was studied. Four h after lipopolysaccharide treatment (20 mg/kg i.p.), the contractile responses to prostaglandin F2alpha in longitudinal muscle strips from the gastric fundus were not different from those in control animals, while the well-known decreased response to noradrenaline in rings of the thoracic aorta was confirmed. Incubation of the tissues with L-arginine did not depress the response to prostaglandin F2alpha in fundus strips of lipopolysaccharide-treated rats. Twelve h after lipopolysaccharide treatment (6.7 mg/kg i.p.), the prostaglandin F2alpha-induced contractions were consistently depressed. The impairment of the prostaglandin F2alpha-induced responses by lipopolysaccharide treatment was not reversed by the nitric oxide synthase inhibitors NG-nitro-L-arginine (L-NNA, 10(-4) M), NG-nitro-L-arginine methyl ester (L-NAME, 3 x 10(-4) M), aminoguanidine (10(-4) M) and L-N6-l-iminoethyl-lysine (L-NIL, 10(-4) M) nor by the cyclooxygenase inhibitor indomethacin (10(-5) M). The impairment was prevented by pretreating the animals with dexamethasone (5 mg/kg i.p.), which had no effect per se on the contractile response to prostaglandin F2alpha. Lipopolysaccharide treatment did not influence the contractile responses to KCl and serotonin. The nonadrenergic noncholinergic relaxant responses to transmural electrical stimulation were not influenced 4 h after lipopolysaccharide treatment but were moderately reduced after 12 h. The results illustrate that the selective impairment of prostaglandin F2alpha-induced contractions in the rat gastric fundus by lipopolysaccharide treatment is not mediated via generation of nitric oxide; downregulation of the prostaglandin F2alpha-receptor by lipopolysaccharide treatment might be involved.

cAMP and purinergic P2y receptors upregulate and enhance inducible NO synthase mRNA and protein in vivo

Adenosine 3',5'-cyclic monophosphate (cAMP) and purinergic P2y receptor agonists upregulate inducible nitric oxide (NO) synthase (iNOS) but inhibit Escherichia coli endotoxin lipopolysaccharide (LPS)- and cytokine-mediated upregulation of iNOS in cultured cells. We examined the effects of cAMP and P2y receptor agonists on the iNOS system in vivo. Intratracheal administration of dibutyryl-cAMP (DBcAMP, 0.1 and 1 mg/kg), a P2y receptor agonist [2-methylthioadenosine 5'-triphosphate (MeS-ATP), 5 mg/kg], or LPS (0.6 mg/kg) to rats 2 h before bronchoalveolar lavage (BAL) increased iNOS mRNA (competitor-equalized reverse transcription-polymerase chain reaction) and iNOS protein (Western blot) in rat alveolar macrophages compared with the effects of sterile phosphate-buffered saline (0.5 ml it). At equal levels of upregulation of iNOS mRNA, 1) LPS, but not DBcAMP or MeS-ATP, upregulated nuclear transcription factor-kappa B (NF-kappa B) and 2) iNOS protein and formation of NO were greater in alveolar macrophages from LPS- and MeS-ATP-treated rats than from DBcAMP-treated rats. Administration of DBcAMP or MeS-AMP 15 min before LPS did not inhibit LPS-induced alveolar macrophage-derived iNOS mRNA, iNOS protein, and NO. Diethyldithiocarbamate (DETC, 5 mg/kg it) inhibited LPS-induced iNOS mRNA but did not affect upregulation of iNOS mRNA produced by the other agonists. We conclude that an LPS-dependent and -independent pathway of iNOS mRNA induction exists in vivo. The former is activated by IPS and most cytokines, is associated with upregulation of NF-kappa B and inhibited by DETC, and elicits an inflammatory response. The latter, activated by DBcAMP and MeS-ATP, is not associated with upregulation of NF-kappa B, inhibition by DETC, or activation of inflammation. The two systems are additive in vivo rather than antagonistic. Speculatively, if the LPS-independent iNOS pathway exists in humans, the iNOS in tissues from patients taking drugs affecting cAMP or P2y receptors may be iatrogenic rather than pathogenetic in origin.

Electron microscopic localization of nitric oxide I synthase in the organ of Corti of the guinea pig

Nitric oxide synthase (NOS) activity has been detected previously in the mammalian cochlea at a light microscopic level. Here we present results of electron microscopic analysis for post-embedding immunoreactivity of neural-type NOS I in the cochlea of the guinea pig. Strong enzyme immunoreactivity was identified in the cytoplasm of inner and outer hair cells. Gold-labeled NOS I antibodies were mainly located in electron-dense areas of the cytoplasm, whereas electron-lucent regions of the receptor cells were nearly free from any immunoreactivity. In both types of hair cells anti-NOS I antibodies were also visible in the cuticular plates, hair bundles and nuclei. Further ultrastructural analysis revealed that the submembranous cisternae of the outer hair cells were nearly free from any reaction product, demonstrating that the whole cytoplasm of this hair cell was not immunoreactive. Other NOS I immunoreactivity was identified in the cuticular plates of the inner and outer pillar cells and in the cytoskeletal elements located in the apical parts of Deiter cells, forming the lamina reticularis or in cytoskeletal-containing regions in basal Deiter cells. Anti-NOS antibodies were visible in the nuclei of various cell types. Our findings suggest that nitric oxide produced by NO I synthase in the organ of Corti may act as a modulator of hair cell physiology during the processes of signal transduction with frequency selectivity.

NOS type-1 mRNA expression and protein localization in spinal autonomic neurons

Autonomic neurons of the rat spinal cord show strong NADPH diaphorase activity and immunoreactivity for nitric oxide synthase (NOS). Here we show mRNA expression of NOS type-1 (neuronal or brain NOS) transcripts in cell bodies of sympathetic preganglionic neurons (SPNs) of the intermediolateral (IML) cell column by non-radioactive in situ hybridization using NOS-I riboprobes. Hybridization signals occurred only in neuronal cell bodies and not outside, in what appeared to be fibers and/or terminals. In preganglionic fibers of SPNs, however, dense axoplasmic immunogold labeling was detected with a monoclonal anti-NOS-I antibody. Expression of NOS-I mRNA in SPN cell bodies and axoplasmic immunolocalization of NOS-I protein suggest that protein translocation is involved in NO-mediated preganglionic control of peripheral targets.

Inhibition of murine macrophage nitric oxide synthase expression by a pivoxil prodrug of antiviral acyclic nucleotide analogue 9-(2-phosphonomethoxyethyl)adenine

The effect of the acyclic nucleotide analogue, 9-(2-phosphonomethoxyethyl)adenine (PMEA, Adefovir), and its (bis)pivaloyloxymethyl ester (bis-POM-PMEA, Adefovir Dipivoxil) on in vitro nitric oxide (NO) production by murine peritoneal macrophages was investigated. Bis-POM-PMEA inhibited in a concentration-dependent manner the formation of NO generated by interferon-gamma and lipopolysaccharide, the IC50 being 15 microM. Suppressed transcription of mRNA for inducible NO synthase (EC 1.14.13.39) resulting in decreased synthesis of NO synthase protein was found. Parent compound PMEA was virtually ineffective.

Use of capillary electrophoresis with laser-induced fluorescence detection to assess messenger ribonucleic acid molecules amplified by the polymerase chain reaction: applications in the cloning of cells

Progressive and selective degeneration of specific classes of neurons occurs in the Alzheimer's disease (AD) brain. Differential vulnerability in this disease is evident even within supopulations that synthesize and release acetylcholine as a transmitter; i.e., basal forebrain cholinergic neurons degenerate but other classes of cholinergic neurons are relatively preserved. The basis for this selective vulnerability is unknown. Studies of differential neuronal vulnerability in AD would be facilitated if cell lines expressing neurotransmitter-specific phenotypes could be cloned from the brain. Capillary electrophoresis (CE) with laser-induced fluorescence (LIF) has been shown to be a sensitive method of detection and quantitation of the DNA products of the polymerase chain reaction (PCR). CE/LIF was combined with the PCR to detect phenotypic messenger RNA (mRNA) molecules, converted to cDNA using reverse transcriptase (RT), in cultures of virally immortalized brainstem progenitor cells produced during establishment of a cloning strategy. RT/PCR methods were developed for detection of the mRNAs for choline acetyltransferase (ChAT), the neuronal, constitutive isoform of nitric oxide synthase (c-NOS), and the growth-associated protein GAP-43, three genes known to be expressed in central cholinergic neurons. A "nondestructive" method of screening cultured cells for their expression of c-NOS was established using depolarization with medium containing 50 mM potassium ion. These approaches were first validated using cultured SN56 (cholinergic) and N1E-115 (c-NOS-positive) neuroblastoma cells, and with primary brainstem cultures. For the cloning of novel cell lines, progenitor cells were isolated from the embryonic day 13 fetal brainstem and were immortalized by transfection with a retroviral vector that confers a temperature-sensitive SV-40 transforming activity and neomycin resistance. Cell colonies surviving in G418-containing media were isolated and cloned by dilution. Clonal cultures were expanded by growth at 33 degrees C, differentiated by switching to a low-serum medium and growth at 39 degrees C, and screened for depolarization-induced accumulation of nitrite in the medium. The subset of putative c-NOS-positive clones (about 4%) were then screened for their expression of mRNAs using RT/PCR in combination with CE/LIF. This screening protocol proved to be powerful in the rapid isolation and phenotypic characterization of immortalized progenitor cells cloned from embryonic rat brainstem.

Antiretroviral agent (R)-9-(2-phosphonomethoxypropyl)adenine stimulates cytokine and nitric oxide production

The immunomodulatory properties of (R)-enantiomer of 9-(2-phoshonomethoxypropyl)adenine ((R)-PMPA), one of the most potent acyclic nucleotide analogs effective against human immunodeficiency virus (HIV), were investigated under in vitro conditions using murine peritoneal macrophages. Remaining without influence on interferon-gamma and interleukin-2 expression, (R)-PMPA dramatically stimulated in a concentration- and time-dependent manner the secretion of tumor necrosis factor alpha (TNF-alpha) and interleukin-10. It also substantially augmented the production of nitric oxide (NO) induced by exogenous interferon-gamma. Inhibitory experiments using neutralizing antibodies against TNF-alpha and/or interleukin-10 demonstrated that these two cytokines are major factors responsible for triggering the underlying mechanism(s) leading to enhanced NO production. The novel findings on the immunomodulatory potential of acyclic nucleotide analogs are discussed in the context of their possible implication in antiviral therapeutic efficacy.

Immunocytochemical and histochemical localization of nitric oxide synthase in the turtle retina

Recent interest in nitric oxide and its relationship to cGMP has produced many attempts to anatomically localize the enzyme synthesizing nitric oxide, nitric oxide synthase. In the retina, numerous previous studies have used the NADPH-diaphorase enzyme activity of nitric oxide synthase as a histochemical method to localize nitric oxide synthase. However, all NADPH-diaphorase activity is not necessarily nitric oxide synthase, because several enzymes have similar biochemical activity. Additionally, various histochemical methods have been used to demonstrate NADPH-diaphorase activity, which makes comparisons between studies difficult. The purpose of this study was twofold. First, we wanted to examine the histochemical labeling of NADPH-diaphorase in the turtle retina to allow comparisons to previous studies. Second, we wanted to compare the histochemical localization of NADPH-diaphorase activity to the immunocytochemical localization of nitric oxide synthase in the turtle retina. Our histochemical localization of NADPH-diaphorase activity and our localization of nitric oxide synthase-like immunoreactivity in the turtle retina both produced similar results. Both the histochemistry and immunocytochemistry consistently labeled photoreceptor inner segments, at least three amacrine cell types, and processes in the inner plexiform layer. In optimized double-labeled preparations, all cells with NADPH-diaphorase activity were also positive for nitric oxide synthase-like immunoreactivity, although some somata in the ganglion cell layer only had nitric oxide synthase-like immunoreactivity. The immunocytochemical localization of nitric oxide synthase in photoreceptors, amacrine cells, and putative ganglion cells indicates that nitric oxide may function at several levels of visual processing in the turtle retina.

Expressional downregulation of neuronal-type NO synthase I in guinea pig skeletal muscle in response to bacterial lipopolysaccharide

We have investigated the expression of neuronal-type NO synthase I (NOS I) and inducible-type NOS II in guinea pig skeletal muscle (diaphragm). Expression of NOS I mRNA and protein was highest in muscle of specific pathogen-free animals, lower in normally bred animals, and lowest in lipopolysaccharide (LPS)-treated animals. NOS II mRNA and protein levels were highest in muscle of LPS-treated animals. Elevated NOS activity in muscle from LPS-treated animals was less susceptible to the NOS I-selective inhibitor N(G)-nitro-L-arginine. Expressional downregulation of NOS I in sepsis may have implications for contractile function of skeletal muscle.

Nitric oxide production by macrophages stimulated by antigen-binding T-cell factors

Contact sensitivity to small molecular weight compounds is accompanied by the production of antigen-specific T-cell factors (TCF) shortly after skin application of the sensitizing agents. In this study, we show that macrophages can be activated by these TCF to generate large amounts of nitric oxide (NO). Incubation of the murine macrophage cell line J774 for 24 h with TCF raised against dinitrofluorobenzene (DNFB) or picryl chloride (PCL) resulted in a nitrite accumulation in the culture medium. Priming of J774 with rIFN-gamma synergistically enhanced stimulation of NO synthesis by DNFB-F and PCL-F. A possible contribution of lipopolysaccharide (LPS) as a contaminant of the TCF was excluded. The enhanced production of NO after stimulation with TCF was accompanied with an increased expression of inducible NO synthase. Inclusion of inhibitors of protein tyrosine kinase and protein kinase C inhibited the TCF-induced NO production by macrophages, indicating the involvement of both protein kinases in the signaling pathway activated by TCF. Since NO is an important biological mediator with many immunoregulatory properties, our results suggest a potential role for increased NO production by macrophages in the elicitation of contact sensitivity to small molecular weight compounds.

Expression of human inducible nitric oxide synthase is regulated by both promoter and 3'-regions

One of the mechanisms underlying the induction of human inducible nitric oxide synthase (hiNOS) by cytokines is transcriptional regulation. However, previous reports suggest that in human cells, the cytokine activation of the hiNOS expression is accompanied by a drastic increase in cellular mRNA and protein content, but by little increment in transcriptional activity. We designed luciferase reporter constructs containing the hiNOS 5'-flanking region and 3'-untranslated region (UTR). The construct containing both 5'-flanking region and 3'-UTR plus its downstream canceled the constitutive activity of the reporter gene, and was more responsive to the cytokines. This is an initial attempt to highlight the important role played by the 3'-region of the hiNOS gene. Our studies provide the evidence that the cooperative interaction between the 5'-promoter and the 3'-region conveys marked induction of the gene in the presence of cytokines.

Bromocriptine is a strong inhibitor of brain nitric oxide synthase: possible consequences for the origin of its therapeutic effects

The ergot alkaloid bromocriptine (BKT) was found to act as a strong inhibitor of purified neuronal nitric oxide synthase (NOS) (IC50 = 10 +/- 2 microM) whereas it was poorly active towards inducible macrophage NOS (IC50 > 100 microM). BKT affects the activation of NOS by calmodulin, as it not only inhibits L-arginine oxidation to NO and L-citrulline but also NADPH oxidation and calmodulin-dependent cytochrome c reduction catalyzed by neuronal NOS. These results suggest that BKT could exert some of its therapeutic effects by interfering with the NOS-dependent formation of nitric oxide and/or superoxide ion in various tissues.

Evaluation of an electrochemical microprobe for direct NO measurement in the rat gastric fundus

Nitric oxide (NO) is an inhibitory nonadrenergic, noncholinergic (NANC) neurotransmitter in the rat gastric fundus and is released upon electrical or pharmacological stimulation of the inhibitory NANC neurons. In this study, it was attempted to measure the release of NO from the rat gastric fundus upon electrical stimulation or administration of nicotine directly via an electrochemical probe (ISO-NO). The system was evaluated by adding exogenous NO. Addition of exogenous NO induced concentration-dependent relaxation of the rat gastric fundus and an increase in the ISO-NO probe baseline current. The concentration of NO detected by the ISO-NO probe was lower than the concentration of NO administered. When no tissue was present, higher concentrations of NO were detected than in the presence of a tissue. In the absence of 95% O2/5% CO2 the concentration of NO detected was highest. Electrical stimulation induced relaxations of the rat gastric fundus which were reduced by NG-nitro-L-arginine methylester (L-NAME). An increase in the ISO-NO probe baseline current was also observed, but this was duc to nonspecific effects as the response also occurred without a tissue present and was not sensitive to L-NAME. Nicotine induced relaxations, which were reduced by L-NAME, but the ISO-NO probe baseline current remained unaltered, even in the presence of L-arginine plus superoxide dismutase. It can be concluded that it is not possible to detect directly the NO release from the rat gastric fundus upon electrical or pharmacological stimulation of the NANC neurons with the ISO-NO probe.

Differences in the localization of the postsynaptic nitric oxide synthase I and acetylcholinesterase suggest a heterogeneity of neuromuscular junctions in rat and mouse skeletal muscles

Recently, nitric oxide synthase (NOS) I has been identified in skeletal muscle fibers, where the enzyme is found to be associated to the sarcolemma by the alpha 1-syntrophin-dystrophin complex. It has, however, been proposed that a substantial proportion of NOS I at the neuromuscular junction (NMJ) is of neuronal origin. We have, therefore, investigated the distribution of NOS I in NMJ of normal rats and mice as well as mdx mice which lack dystrophin and, consequently, NOS I in the sarcolemma region by enzyme histochemical and immunohistochemical techniques. Sites of NOS I accumulation, evident at NMJ of healthy animals, were absent in mdx mice, indicating a predominantly, if not exclusively, postsynaptic localization of NOS I at NMJ. Moreover, simultaneous demonstration of acetylcholinesterase (AChE) activity revealed a heterogeneity of NMJ in rat and mouse skeletal muscles: type I showed only AChE activity and was found to predominate; type II was spatially separated from the AChE-positive NMJ, occurred less frequently and contained both AChE activity and NOS I. These data suggest that type II NMJ are provided with additional regulatory mechanisms, such as free radical signaling by the NOS I-derived NO which may exert modulatory effects on the choline acetyltransferase/ACh/AChE pathway. Furthermore, type II may represent those NMJ where recently glutamate-gated NMDA-type Ca2+ channels have been described, which in analogy to those in the nervous system may serve also in skeletal muscle fibers as NOS I activators.

Nitric oxide and fibroblast growth factor in autonomic nervous system: short- and long-term messengers in autonomic pathway and target-organ control

The freely diffusible messenger nitric oxide (NO), generated by NO synthase (NOS)-containing "nitroxergic" (NO-ergic) neurons, is unique among classical synaptic chemical transmitters because of its "non-specificity", molecular "NO-receptors" (e.g. guanylyl cyclase, iron complexes, nitrosylated proteins or DNA) in target cells, intracellular targeting, regulated biosynthesis, and growth factor/cytokine-dependence. In the nervous system, expression of NOS is particularly intriguing in central and peripheral autonomic pathways and their targets. Here, anatomical and functional links appear to exist between NOS, its associated catalytic NADPH-diaphorase enzyme activity (NOSaD) and fibroblast growth factor-2 (FGF-2), a pleiotropic cytokine with mitogenic actions, suggesting mutual "short- and long-term" actions. Several recent studies performed in the rat sympathoadrenal system, an anatomically and neurochemically well-defined autonomic pathway with target-specific functional units of sympathetic preganglionic neurons (SPNs) in the spinal cord, provide evidence for this hypothesis. The NO and cytokine signals may interact at the level of gene expression, transcription factors, post-transcriptional control or second messenger cross-talk. Thus, unique biological roles of FGF-2 and the NO system are likely to exist in neuroendocrine actions, vasomotory perfusion control as well as in neurotrophic actions in sympathetic innervation of the adrenal gland. In view of their anatomical co-existence, functional interplay and synchronizing effects on neuronal networks, multiple roles are suggested for both "short- and long-term" signalling molecules in neuroendocrine functions and integrated autonomic target organ control.

Substrate binding and calmodulin binding to endothelial nitric oxide synthase coregulate its enzymatic activity

Endothelial nitric oxide synthase (NOS) is a constitutively expressed flavin-containing heme protein that catalyzes the formation of NO from L-arginine, NADPH, and molecular oxygen. We purified bovine endothelial NOS from transfected embryonic kidney cells by conventional chromatographic techniques and characterized the activity of the detergent-solubilized enzyme. Endothelial NOS displays a much lower specific activity of NO synthesis (143 +/- 11 nmol NO/min/mg enzyme) than the constitutive neuronal NOS or inducible NOS isoforms. Like the neuronal isoform, endothelial NOS requires binding of Ca2+/calmodulin to achieve Vmax NO synthase activity; however, we observed a basal level of NO synthesis even when Ca2+/calmodulin was omitted and 0.5 mM EDTA was present in the assay solution. Moreover, endothelial NOS demonstrates a high-affinity bonding interaction with calmodulin such that the enzyme as purified has a NO synthase activity at about 80% of Vmax. We also observed a more than twofold increase in NADPH consumption by endothelial NOS when it was coupled to arginine oxygenation as opposed to when oxygen is activated in the absence of substrate. Substrate binding was also shown to stimulate heme reduction in the absence of added calmodulin. Thus, the enzymatic synthesis of NO from L-arginine by endothelial NOS appears to be partially regulated by binding of both calmodulin and substrate. These findings for endothelial NOS represent a significant departure from the enzymatic properties of the other constitutive NOS isoform, neuronal NOS, and we interpret this result in terms of the physiological implications.

Tolerance to lipopolysaccharide-induced increase in vascular permeability in mouse skin

We investigated whether tolerance develops to the lipopolysaccharide-induced increase in vascular permeability of mouse skin on pretreatment with Salmonella typhimurium lipopolysaccharide. Lipopolysaccharide-induced plasma extravasation was assessed by determining Pontamine sky blue dye accumulation in the skin where lipopolysaccharide was injected s.c. 2 h previously. When mice were pretreated with lipopolysaccharide (0.15 mg/kg i.p.), the dye leakage induced by s.c. challenge with lipopolysaccharide (400 micrograms/site) was significantly, inhibited for 2-24 h after pretreatment, indicating the development of lipopolysaccharide tolerance. At 4 h after lipopolysaccharide (0.15 mg/kg i.p.), the dose-response curve of dye leakage against the challenge dose of lipopolysaccharide shifted about 2-fold to the higher dose. The dye leakage induced by lipopolysaccharide was inhibited by pretreatment with lipopolysaccharide in a dose-dependent manner (0.05-0.15 mg/kg i.p.). Lipopolysaccharide tolerance was not seen in adrenalectomized mice. When mice were pretreated with lipopolysaccharide and NG-nitro-L-arginine methyl ester (L-NAME), a nitric oxide (NO) synthase inhibitor, at the same time, the hyporesponsiveness to lipopolysaccharide challenge disappeared. However, L-NAME was ineffective to inhibit the development of lipopolysaccharide tolerance when administered 24 h after lipopolysaccharide pretreatment or just before the lipopolysaccharide challenge. Tumor necrosis factor-alpha and interleukin-1 alpha but not interleukin-6 induced a similar hyporesponsiveness to lipopolysaccharide. These results suggest that tolerance develops to the lipopolysaccharide-induced increase in vascular permeability in mouse skin after a single lipopolysaccharide administration and that endogenous glucocorticoids and NO are necessary for induction of lipopolysaccharide tolerance. Hyporesponsiveness induced by lipopolysaccharide pretreatment may be mediated by production of some cytokines such as tumor necrosis factor-alpha or interleukin-1 alpha.

Selective inhibitors of nitric oxide synthase (NOS) implicate a constitutive isoform of NOS in the regulation of interleukin-1-induced ACTH secretion in rats

Nitric oxide synthase (NOS) exists in at least three distinct isoforms: an inducible NOS (NOS II), and two forms which are constitutively expressed-brain NOS (NOS I) and endothelial NOS (NOS III). We have previously shown that the NOS inhibitor, N(ω)-nitro-L-arginine methyl ester hydrochloride (L-NAME), markedly potentiates and prolongs the increase in plasma adrenocorticotropin (ACTH) concentrations produced by the intravenous injection of interleukin-1β (IL-1β) in the rat. However, the mechanism of action of L-NAME is unknown. The purpose of the present study was to determine the effects on IL-1β-induced ACTH secretion in the rat, of several NOS inhibitors, whose selectivity for the different NOS isoforms has been well characterized, and which lack the muscarinic receptor antagonist properties that have been reported for L-NAME. Subcutaneous (sc) pretreatment with L-NAME (50-300 μmol/kg) produced the expected pronounced exacerbation of the ACTH response to IL-1β. This effect was mimicked by N(G)-nitro-L-arginine, which preferentially inhibits constitutive forms of NOS. In contrast, aminoguanidine, a selective inducible NOS inhibitor at doses up to 3×1.8 mmol/kg, was without effect, suggesting that it is a constitutive form of NOS that regulates the ACTH response to IL-1β. Selective inhibition of brain NOS using either 7-nitro-indazole (administered intraperitoneally) or L-NAME (administered intracerebroventricularly) did not significantly alter ACTH concentrations after IL-1β. Collectively, these data indicate that NO restrains the ACTH response to IL-1β, and that the NO responsible for this effect is generated by a constitutive, most probably endothelial, isoform of NOS.